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Possible Xenolith and Xenocrysts of Olivine in the Asuka-881371 Angrite Collected from Antarctica. K. Yanai, Environmental

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Possible Xenolith and Xenocrysts of Olivine in the Asuka-881371 Angrite Collected from Antarctica. K. Yanai, Environmental Lunar and Planetary Science XXXIV (2003) 1205.pdf Possible Xenolith and Xenocrysts of Olivine in the Asuka-881371 Angrite Collected from Antarctica. K. Yanai, Environmental and Planetary Geosciences, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan. E-mail: [email protected] Angrites: Angrite had been known as one large-olivine single grains. Small fragment of the oldest rocks coming from the space. consisting of fine olivine aggregate is only Four and more meteorites had been recognized in the A-881371 angrite. The identified as angrite, such as Angra dos aggregate at bottom of the thin section(Fig. Ries(1.5kg, fall in Brazil 1869[1]), Lewis 2(b)) consists of fine-grained and almost Cliff 86010 (6.9g, find in Antarctica equal granular olivine grains which are 1986[1]), Lewis Cliff 87051(0.6g, find in under 0.1mm in size. It is dunite for its Antarctica 1987[2]), Asuka-881371[3] and mineral assemblage. Compositions of others. In the all meteorites, angrite aggregate olivine in the dunite and large meteorites are very unique specimen for olivine grains are nearly homogeneous Fo86 their oldest age, mineral assemblages such with little variation of core(Fo90Fa10) to as Ca-pyroxene(fassaite) and Ca- rim(Fo81Fa19). Olivine of the dunite and olivine(kirschsteinite), and showing their large grains are quite differ from those of unique textures of differences in each olivine in host rock, because most olivine in angrite. host rock are more fine grain and very wide Asuka-881371 angrite: Asuka- compositional range from Fo3Fa97 to 881371(A-881371) meteorite collected from Fo73Fa27. Therefore, olivine aggregate Antarctica by the Japanese party in 1988 (dunite inclusion or fragment) and large field season. The A-881371 is small stone olivine grains might be both xenolith and of 11.27 grams in weight. It is rounded stone xenocrysts respectively in the host Asuka- almost completely covered with dull black 881371 angrite. If they are xenolith and/or fusion crust. Pale green, and some large xenocrysts in angrite A-881371, it is very grains of olivine crystal are seen on the unique occurrences because angrite is the exposed interior surface(Fig. 1). The A- oldest aged igneous rocks coming from the 881371 have been identified and classified space. as one of the angrite group meteorite which is one of quite unique type of meteorite References: [1] Grady M. M. (2000) belong to achondrites, and it shows the Catalogue of Meteorites, 689p. [2] oldest age almost 4560Ma in all meteorites. Lindstrom M. M. (1989) Antarctic As the thin section (Fig. 2(a)) the A-881371 Meteorite Newsletter 12-1, 22. [3] Yanai angrite is one of igneous rocks showing fine K. and Kojima H. (1995) Catalog of grained, an unbrecciated and typical Antarctic Meteorites, 230p. ophitic(doleritic) texture with euhedral plagioclase, intergranular pyroxene and olivine with opaques and spinel. The A- 881371 angrite also contains several unique- large olivine grains and fine-grained olivine aggregate in the host matrix. In the host rock, pyroxene is the most abundant mineral and is relatively strong plecochroic halo, from near colorless in the core to brown in the rim. Most pyroxene show hight Ca content over 50% Wo and wide ranged of En and Fs components. Compositions of pyroxene are En0-28.7Fs18.0-50.2Wo48.1- 54.7, therefore almost all pyroxene is fassaite. Olivine shows very wide compositional range of Fo2.8-73Fa97.2-27 containing 0.1-1% Ca. So olivine is Ca- olivine kirschstinite. Plagioclase is Fig. 1. Asuka-881371 angrite, 11.27g. remarkably and virtually pure anorthite over The specimen is a rounded stone, almost An99. completely covered with a dull-black fusion Lithic fragment and large olivine crust. Relatively large and pale green grains: There are two types of unique porphyritic olivine crystals are seen on the occurrences of olivine in this angrite. One exposed interior surface. Scale cube is is fine-grained olivine aggregate such as 1cm. lithic fragment or inclusion. Others are Lunar and Planetary Science XXXIV (2003) 1205.pdf (a) Fig. 3. Composition diagrams for pyroxene(fassaite), plagioclase(non- maskelynized) and Ca-olivine (b) (krischsteinite) *: High-Mg olivine(Fo90- 81) in dunite inclusion or fragment (xenolith?) and large olivine grains (xenocryst?). Fig. 2. (a): Photomicrograph of the thin section of the Asuka-881371 angrite. The thin section shows an unbrecciated and typical ophitic texture with plagioclase, intergranular pyroxene(fassaite) and Ca- olivine(kirschsteinite) including several large olivine grains. A fine-grained olivine aggregate (dunite inclusion or fragment) in bottom. Long dimension is 12mm. (b): Dunite inclusion or fragment (possible xenolith?) consisting of fine- grained olivine with high Fo compoment. Long dimension 1.4mm..
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